Abstract This project aims to develop a novel system, Spines InvestigatorTM, for performing automated four-dimensional (4D) quantitative analysis of changes in dendritic spine morphology on three-dimensional (3D) microscopic images acquired with in vivo multiphoton fluorescence microscopy at different time points. The role of dendritic spines is one of the most active and important areas of neuroscience research. Plasticity of dendritic spine morphology plays a crucial role throughout life - in development, aging, as well as in learning and memory. Also, many complex brain diseases, including autism spectrum disorders, schizophrenia, Down syndrome, Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and stroke, are characterized by dendritic spine pathology including abnormal dendritic spine density and morphology, dendritic spine loss, and aberrant dendritic spine plasticity. While it is not possible to study the plasticity of dendritic spine morphology in the human brain in vivo, it is possible in mouse models of complex human brain diseases. However, the study of these mouse models remains a tedious and cumbersome endeavor because tools for automated 4D dendritic spine quantitative analysis are not available. Critical steps that are currently performed manually in such investigations may lead to faulty and irreproducible results, which does not conform with NIH's rigor and transparency policy. Spines Investigator will help solve this untenable situation with a number of distinct innovations. Specifically, Spines Investigator will comprise novel technology that enables the automated comparison of dendritic spine morphology on 3D images acquired with in vivo multiphoton fluorescence microscopy in the brain of a mouse at precisely the same site at different time points. It will also enable new research that combines 4D in vivo quantitative analysis of changes in dendritic spine morphology with the analysis of amyloid plaques (in Alzheimer's disease), as well as analysis of microglia and astrocytes . To create this new solution for automated 4D in vivo quantitative analysis of dendritic spine morphology, Spines Investigator will build upon our Neurolucida360 technology developed during Phase II (SBIR Fast-track Grant MH093011). We will develop Spines Investigator as a tested, validated, supported and fully documented system. The benefit for the neuroscience research community, pharmacological and biotechnological research and development, and society in general will be to better understand the critical role of the plasticity of dendritic spine morphology in the brain under various physiological and pathological conditions. In particular, this will result in an improved basis for developing novel treatment strategies for complex brain diseases.